Population description

A Population refines a Universe by specifying the geographic area and time period to which its units belong. Units may be people, households, organizations, events, or other objects of interest.

This follows the Unit Cascade in DDI Common Core, version 1 (p. 5): Unit Type identifies a general kind of unit, Universe restricts it by shared characteristics, and Population adds geographic and temporal scope. For example, “persons” is a Unit Type, “women aged 18 or older” is a Universe, and “women aged 18 or older residing in the United States during 2025” is a Population.

Use UniverseReference to identify the defining Universe, GeographyOfPopulation to describe its geographic scope, and TimePeriodOfPopulation to describe its temporal scope. Multiple time periods can describe non-contiguous intervals. These properties are optional in the current model; a description aligned with Common Core should nevertheless establish both the geographic and temporal scope.